It has often been writen that John Winthrop (1588-1649) was a Justice of the Peace in England, that statement frequently being amplified by the "fact" that he presided over his first court at the age of eighteen. The latter claim actually is based on Winthrop presiding over a manorial court session, which he was empowered to do not by being a Justice of the Peace but by virtue of his status as lord of Groton manor.

"JPs" were members of the Commission of the Peace, appointed by the crown and no documentation of Winthrops being on the Commission has previously been provided. Employed by the Winthrop Papers Project, Ms Sue Sadler was able to search the appropriate runs of documents in the Public Record Office and as a result has been able to establish Winthrop's tenure on the Commission for Suffolk.

According to J. H. Baker, An Introduction to English Legal History, third edition (London, 1990), the system operated as follows: "At intervals a commission of the peace was drawn up for each county, listing the substantial knights and gentry of the area and taking care to include the sages et apris de la leye, charging them both to keep the peace and to enquire into, hear and determine' a long list of crimes, ranging from felonies to economic offences and sorcery. The first of these 'charges' imposed an individual police responsibility on each justice; justices could arrest suspects and commit them to gaol, and could require anyone to give surety for keeping the peace.... The second was in effect a general commission of oyer and terminer to any two or more of the justices (with a 'quorum' of lawyers), and empowered the justices collectively to hold their sessions of the peace. Directed by statute to be held at four seasons (Michaelmas, Epiphany, Easter and the Translation of St Thomas), these were known as the general quarter sessions of the peace. The jurisdiction of quarter sessions was in theory virtually coterminous with the criminal side of the assizes, but in practice inferior." Quorum meant "of whom"; in the judicial part of the commission the list of justices was followed by a quorun clause.

John Winthrop was appointed to the Commission of the Peace for Suffolk in 1615 and seems to have served continuously till omitted at some point in the mid-1620s, probably the accession of Charles I. He was reinstated in June of 1626 and continued to be listed through 1631 – despite his having left England for Massachusetts.